feat: presence on tickets

A ticket's page beats every 10 s (touchPresence / PUT
/issues/{issue}/presence, with whether I am editing) and gets back who
else is there, editing ones first. Entries live in Redis, one hash per
ticket, and fade after 30 s without a beat; leaving the page takes me out
at once. Only for people who can read the ticket; never in the event
store. The ticket shows «Bob está editando · Ana también lo está viendo»
above its title; editing means a ticket or comment form is open or a
comment is being written.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/infrastructure/src/lib/adapters/graphql-presence.gateway.ts

@@ -0,0 +1,25 @@
+import { inject, Injectable } from '@angular/core';
+import { Presence, PresenceGateway } from '@task-stream/web/tracker/domain';
+import { map, Observable } from 'rxjs';
+import {
+  LeavePresenceDocument,
+  TouchPresenceDocument,
+} from '../graphql/generated';
+import { GraphqlOperations } from './graphql-operations';
+
+@Injectable()
+export class GraphqlPresenceGateway extends PresenceGateway {
+  private readonly gql = inject(GraphqlOperations);
+
+  touch(issueId: string, editing: boolean): Observable<Presence[]> {
+    return this.gql
+      .mutate(TouchPresenceDocument, { issueId, editing })
+      .pipe(map((d) => d.touchPresence));
+  }
+
+  leave(issueId: string): Observable<void> {
+    return this.gql
+      .mutate(LeavePresenceDocument, { issueId })
+      .pipe(map(() => undefined));
+  }
+}